Rejoice always, pray continually, give thanks in all circumstances; for this is God's will for you in Christ Jesus.Is this even possible? Aren't there times when rejoicing or giving thanks would be inappropriate, like in the wake of a natural disaster? Is it reasonable to command people to be in constant prayer? What would that even look like?
I don't know if any of these are possible, but I think there's a deeper principle at work here, and it's this: Your character can exceed your circumstances. Don't let the circumstances of your life bring you down to the pit, or shut your mouth from prayer, or make you embittered and ungrateful. No matter what comes your way, the way you respond is entirely up to you. Rejoicing, prayer, and thankfulness are always a conscious choice. You don't just fall into those responses by accident; you do them on purpose.
It is God's will for you that your character be determined by the power of Christ in you rather than on your instinctive reactions to the various circumstances of your life. You might say that your natural response to your circumstances is what is true, and to force yourself to respond another way is hypocritical. Not so. If you follow Jesus, what is truest about you is Christ in you. Jesus Christ is what is most true of you. Not your sin. Not your past. Not your temper. Not your attitude. Not your instinctive reactions to your circumstances. Through faith in Christ, you are no longer a "natural" person, but a "becoming-supernatural" person by the power and presence of the Holy Spirit. The old is gone, crucified with Jesus, and the new is here, resurrected with Jesus. You are new, through faith in Christ.
You have power, in the Spirit, to rise above your "natural" reactions and instincts. I'm not saying it's easy. I'm not saying you can change overnight. But you can learn to walk in the Spirit--and to rejoice always, to pray continually, and to give thanks no matter what--the same way you learned to walk as a toddler. By falling down a lot, and getting back up.
I'm a pastor, and I'm still learning to walk. It's hard. Sometimes I wake up on the wrong side of the bed, which really just means I'm choosing to be a frustrated, mean-spirited, downcast jerk like I am today. I don't always remember these things, but that doesn't make them any less true. My character can exceed my circumstances, but only as I lean into the power of Christ within me through the presence of the Holy Spirit. The same goes for you. And be encouraged, because you'll learn to walk someday.